A. Field of the Invention
This invention relates to semiconductor devices and in particular to a sealing coating for protecting the device from contamination.
B. Prior Art
In the fabrication of integrated circuits, a wafer of monocrystalline silicon is variously etched and subjected to diffusion or implantation of controlled concentrations of impurities to establish desired topology and various N-type, P-type and insulating regions which form the multiplicity of active and passive semiconductor structures. A final step in fabricating an operating device is the laying on of an aluminum film pattern to establish metallic interconnections between the structures such that a desired circuit is produced. Portions of this pattern are terminated at bonding pads to which wires are later attached; the wires extending to terminal pins of the completed package.
The device at this stage of fabrication is functionally complete and could, with appropriate mechanical support and termination, operate as part of an electronic assembly such as a television receiver, calculator, etc. Its usefulness would be shortlived, however, because environmental contamination would profoundly affect the character of the semiconductor junctions and the regions abutting them. Specifically, sodium ions and moisture present in the atmosphere would easily diffuse into the silicon and radically change the electrical characteristics of the device. Such influences would cause the demise of the device in only hours at ordinary temperatures.
It is, therefore, common practice to deposit a layer of silicon dioxide (SiO.sub.2) which in its amorphous form is essentially glass. This layer protects the underlying silicon from contact with atmospheric gases and vapor. Furthermore, it protects the wafer surface and its metalization against abrasion and gross contaminants such as dust particles which might be introduced during the cutting of the wafer into individual chips.
If the chip is then housed in an hermetic seal, the SiO.sub.2 passivating layer has usually been sufficient. The chip has remained protected against residual contamination within the enclosure which is in turn protected against further contamination by the seal. However, hermetic sealing is relatively costly and thus for most consumer and industrial products, the completed glass sealed chips are encapsulated in plastic following the bonding of the wire leads. The plastic offers adequate mechanical protection and serves to exclude gross contaminants. However, the plastic is semi-permeable and moisture has diffused through it over periods of time. The moisture has also transported metallic ions, typically sodium. The layer of passivating glass has protected the device as described above against these contaminants over a period ranging from weeks to years depending on temperature, electrical biases and the thickness and constitution of the glass itself.
Glass containing about 3 to 10 percent, typically 5% to 7%, of phosphorous has been enhanced in its ability to trap sodium ions. However, these ions over a lengthy period have diffused through the glass in sufficient numbers to alter the electrical characteristics of a device. Such diffusion rates are enhanced at higher chip temperatures and by electric fields which are produced by operating potential differences between various regions of the chip surface.
Accordingly, an object of the present invention is protecting the major surface of a semiconductor device from moisture and metallic ions present in the atmosphere.